Fix wrong device being considered selected

ag/31194237 has an incorrect condition check where we add selected
routes. The check is supposed to be gated behind the flag, but instead
the entire logic on added sorted routes was gated instead.

Bug: 394165966
Test: atest, local test
Flag: com.android.media.flags.enable_output_switcher_session_grouping
Change-Id: I285c6cf3cc6864e199f0067116e14d10c1464f94